A hydraulic working machine such as a hydraulic excavator is equipped with a hydraulic drive system including a plurality of hydraulic actuators, a hydraulic pump, etc., and conducts various required operations while driving the plurality of hydraulic actuators by the hydraulic pump. The hydraulic drive system generally used comprises a variable displacement hydraulic pump, a plurality of hydraulic actuators driven by the hydraulic pump, a plurality of flow control valves of the center bypass type for controlling the driving of the hydraulic actuators, and a center bypass line connecting the center bypasses of the flow control valves in series. As disclosed in JP, A, 125921, such a hydraulic drive system also comprises a flow resisting mean, e.g., a fixed throttle, disposed downstream of the center bypass line for generating a negative control pressure in the center bypass line, a pressure sensor for detecting the negative control pressure generated in the center bypass line, a controller for calculating, based on the detected value of the pressure sensor, the target displacement volume of the hydraulic pump (i.e., the tilting amount of a swash plate) in accordance with a preset characteristic and then outputting an electric signal corresponding to the calculated value, and a regulator driven by the electric signal for controlling the displacement volume of the hydraulic pump.
The center bypass of each of the flow control valves is fully opened when the associated flow control valve is in a neutral position, and is restricted gradually as the valve is shifted from the neutral position. When all the flow control valves are in neutral positions, i.e., any of the hydraulic actuators is not driven, a hydraulic fluid delivered from the hydraulic pump is passed at a full flow rate through the center bypass line, and the negative control pressure detected by the pressure sensor is maximized. The controller calculates the minimum target displacement volume in accordance with the preset characteristic, and the hydraulic pump is controlled so that the displacement volume (i.e., the delivery rate) thereof is minimized. In an attempt to drive one hydraulic actuator, for example, when the corresponding flow control valve is operated, the center bypass of the operated valve is restricted to reduce the flow rate passing through the center bypass line, and the negative control pressure detected by the pressure sensor is also reduced. Therefore, the target displacement volume calculated by the controller is increased in accordance with the preset characteristic, whereupon the hydraulic pump increases the target displacement volume and delivers the hydraulic fluid at a flow rate enough to drive -the hydraulic actuator.